1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor memory device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics. A display device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
Dynamic random access memory (DRAM) is widely used as a main memory device of a semiconductor device. DRAM has high degree of integration and large storage capacity per unit area. However, since DRAM is a volatile memory, regular refresh operation is required to hold stored data for a long period.
In recent years, use of an oxide semiconductor for a transistor in a memory cell included in DRAM is suggested. Since the off-state current of a transistor including an oxide semiconductor in a channel formation region can be low, the interval between refresh operations can be significantly increased. Thus, power consumption of DRAM can be reduced (see Patent Document 1).